The chemical composition of gasifier syngas predominantly depends on the particular gasification technology and feedstock characteristics. For example, oxygen blown gasifiers will produce a concentrated syngas with mainly H2 and CO, while air blown gasifiers will produce a nitrogen rich syngas. Where the gasification feedstock is petroleum coke, vacuum residual oil, or tar, the syngas will typically have a relatively high H2S content. On the other hand, where the gasification feedstock is coal, the sulfur content in the syngas may vary considerably as lignite and brown coal will generally produce low sulfur syngas and bituminous coal will typically lead to high sulfur syngas.
Regardless of the particular feedstock, gasification plant syngas will generally contain predominantly H2 next to CO2, CO and H2S. When carbon capture is required, one or more CO shift reactors can be implemented to convert CO to H2 and CO2, thus producing a syngas with a relatively high CO2 content, typically greater than 40 mole %. In most cases, the H2S content in syngas varies from 0.05 to over 0.1 mole % (depending on feedstock). Therefore, the CO2 to H2S molar ratio in the syngas is very high (e.g., greater than 100), which poses a substantial problem for sulfur removal that relies on a downstream Claus unit. For example, even where an acid gas removal unit uses an H2S selective solvent, the solvent will co-absorb substantial quantities of CO2 and therefore in almost all cases fail to produce an H2S rich acid gas that is suitable for use in a Claus unit.
Since the Claus unit is a combustion unit, the acid gas feed to a Claus unit must have an H2S concentration of at least 25 mol % along with sufficient heating values for the sulfur conversion reaction. Even higher H2S concentrations are required where the acid gas also contains ammonia and BTEX (benzene, toluene, and xylenes) components. Thus, for most IGCC power plants that are integrated with a Claus unit, the H2S content in the acid gas must be at least 40 mol % to ensure complete ammonia destruction. If the H2S content in the acid gas is too low, additional processing steps are required, such as supplemental fuel gas firing, preheating acid gas, and combustion air and oxygen enrichment. These additional processing steps are costly and can seldom be justified. Alternatively, the acid gas can be concentrated in certain plant configurations by an enrichment process that can be integrated to the acid gas removal unit. Examples for such enrichment units are shown in U.S. Pat. No. 7,635,408 to Mak et al. Here, several fractionation columns are used and the acid gas product is recycled to raise the acid gas content. While these processes are quite selective at low pressure (e.g., 10 psig) operation, they generally fail to produce an acid gas rich in H2S at high pressures such as those encountered in a gasification plant. This and all other extrinsic materials discussed herein are incorporated by reference in their entirety. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
Most typically, syngas is produced at pressures between 400 and 900 psig. Under these conditions, an H2S selective solvent such as DMEPG (dimethyl ethylene polyglycol) provides about 6 to 8 times higher solubility for H2S as compared to CO2. However, such low selectivity is almost always entirely insufficient to produce an acid gas product suitable for a Claus plant, especially where the syngas has a high CO2 to H2S ratio as the solvent circulation required to meet sulfur specifications must be sufficiently high which tends to co-absorb significant amounts of CO2, subsequently diluting the H2S content in the acid gas to the Claus sulfur plant.
Consequently, although many configurations and methods for acid gas removal from syngas are known in the art, all or almost all of them suffer from various disadvantages. Therefore, there is still a need to provide methods and configurations for acid gas removal, especially for treatment of syngas to produce a concentrated acid gas stream suitable as feed for a Claus plant.